1. Field of the Invention
The invention relates to a motor control apparatus that drives a motor and a power steering system using the motor control apparatus.
2. Description of the Related Art
In a power steering system, rotation of a motor is transmitted to a steering shaft to assist steering. To control such a motor, an inverter and drivers for the inverter are used. The inverter includes a bridge circuit with a plurality of sets of an upper arm and a lower arm connected together in series between a positive electrode and a negative electrode of a DC power supply. A middle point between the upper arm and the lower arm in the inverter is connected to each phase coil of the motor. The upper arm and the lower arm are composed of switching elements. The drivers control and turn the switching elements on and off in accordance with control signals. Operations of turning the switching elements on and off allow phase voltages to be supplied to the respective phase coils of the motor, and the motor is thus rotated.
In a conventional circuit configuration, voltage sensors that detect a voltage are each provided at the middle point between the upper arm and the lower arm in order to monitor the corresponding phase voltages of the motor. For example, see Japanese Patent Application Publication No. 2014-135866 (JP 2014-135866 A).
The drivers control and turn the switching elements in the inverter on and off by selectively supplying a predetermined first voltage and a second voltage lower than the first voltage to the switching elements. The second voltage, which is lower than the first voltage, may be based, for example, on a negative voltage lower than a ground voltage. Consequently, the second voltage can be made lower than the first voltage, enabling, for example, enhancement of switching characteristics and the use of normally-on transistors as the switching elements.
As described above, when the negative voltage is used to control the switching elements, it is preferable to monitor a circuit supplying the negative voltage. However, when a sensor configured to monitor the circuit supplying the negative voltage is added besides the voltage sensors detecting the phase voltages, the configuration of the motor control apparatus is complicated.